1. Field of the Invention
The present invention relates to an antenna system in which millimeter waves or sub-millimeter waves having a frequency exceeding 100 GHz are used to reduce the influence of sunlight collected in the vicinity of a focal point of an antenna, and relates to a method for manufacturing such an antenna system.
2. Description of the Related Art
FIG. 7 is an illustration showing the structure of a conventional antenna system. In FIG. 7, a reference numeral 1 denotes an antenna panel; a reference numeral 2 denotes a coating film painted in white or with a semi-gloss on the front surface of the antenna panel 1; and a reference numeral 3 denotes an assistant reflecting mirror disposed on the focal portion of the antenna panel 1. Incident radio waves of millimeter waves or sub-millimeter waves are focused on the assistant reflecting mirror 3 by the antenna panel 1. On the other hand, since sunlight entering the antenna panel 1 is as short as 1 xcexcm or less in terms of its wavelength, as compared with the millimeter waves or the sub-millimeter waves, it is irregularly reflected by the front surface of the coating film 2 painted in white or with a semi-gloss, and is not focused on the assistant reflecting mirror 3.
However, if the frequency used in this kind of antenna system is higher, there is a problem that the radio waves of millimeter waves or sub-millimeter waves passing through inside the coating film 2 are attenuated, to thereby lower the antenna sensitivity or to degrade the observed signals to noise ratio.
In order to solve such a problem, in the related art, there is an antenna system disclosed, for example, in Japanese Patent Application Laid-Open No. 62-131611. FIG. 8 is a schematic sectional view of a conventional antenna system disclosed in the same gazette. In FIG. 8, a reference numeral 1 denotes an antenna panel and a reference numeral 4 denotes fine bumps and dips formed on one surface of the antenna panel 1. The object of this related art is to manufacture an antenna panel which has a diameter of about 50 cm and which can be used without painting.
This antenna panel 1 is manufactured as follows: very small grooves are formed on the front surface of a flat plate previously, though they may be formed later, and then the flat plate is bent and deformed to form the antenna panel 1. In this manner, the elimination of the coating film 2 makes it possible to use the antenna panel 1 even in a high frequency. Further, the fine bumps and dips formed on the front surface of the antenna panel 1 restrain the light reflected by the antenna panel 1 from being collected on a light collecting portion, which results in preventing an assistant reflecting mirror disposed at the light collecting portion from rising in temperature.
The aforementioned gazette discloses that the fine bumps and dips on the front surface of the antenna panel 1 are formed by making scratches in the shape of straight lines or curved lines on the front surface of the antenna panel 1, or by blasting or etching the front surface. Here, the scratches in the shape of straight lines or curved lines are mechanically made by an abrasive paper, an abrasive cloth, a grinder or the like.
FIG. 9 is a graph to show the sunlight reflecting characteristics of a polished surface of a flat plate sample which is made of aluminum (Al) and mechanically polished, and FIG. 10 is an illustration to show a method of measuring the reflecting characteristics. As shown in FIG. 10, the flat plate sample is disposed just opposite to the sun and the reflecting characteristics is measured with an illuminator which is provided with a lightproof cylinder, so that the sunlight is detected only from the front, by changing the angle of the illuminator with respect to the flat plate sample is changed. The surface roughness of the #40 polished product is 4.4 xcexcm RMS (Root Mean Square) and as shown in FIG. 9, the illuminance is very high near the angle of 0 degree and the sunlight is not scattered greatly. Here, the character #40 designates the grain size of abrasive grains and the abrasive grain size becomes smaller as the number is larger.
Theoretically, the extent to which the reflecting scattering characteristics is required is determined as follows. FIG. 11 is a characteristic graph to show the relationship between the ratio of scattered light collected on an assistant reflecting mirror (450 mm) which is in a focal portion and the scattering angle (half width having a half of a peak value), in the case of a large-size millimeter-wave parabolic antenna system which is of the order of 10 m. In this case, energy entering a parabolic antenna is 1xc3x97105 W when the parabolic antenna is just opposite to the sun. When this energy is multiplied by the collecting ratio of the sunlight and the absorptivity of the assistant reflecting mirror, the temperature rises by about 100xc2x0 C. if the light collecting ratio is 0.015. Such temperature rising is considered to be a limit in use, though not sufficient, if time for the parabolic antenna to be just opposite to the sun is not so long. As shown in FIG. 11, the scattering angle (half width) is 55 degrees when the light collecting ratio is 0.015 and thus the scattering angle needs to be larger than the above value of 55 degrees.
In the sample shown in FIG. 9 which is mechanically polished, the scattering angle, that is, the half width having a half of the peak value is 10 degrees and the light collecting ratio at this time is 0.3, so according to a similar calculation, the temperature rises up to 2,000xc2x0 C. Since this temperature exceeds the melting point of aluminum, means of polishing is not applicable to a surface processing method of the parabolic antenna which is of the order of 10 m.
On the other hand, in the sunlight reflecting characteristics of a flat plate sample which is made of aluminum and whose surface is subjected to blasting, the surface subjected to blasting has a surface roughness of about 0.4 xcexcm RMS and shows a scattering angle larger than 55 degrees. Further, in the sunlight reflecting characteristics of a flat plate sample which is made of aluminum and whose surface is etched, the etched surface has a surface roughness of about 1.2 xcexcm RMS and shows a large scattering angle and good scattering characteristics. However, the surface subjected to an electrolytic polishing or a chemical polishing becomes very smooth in surface irregularity and is not applicable.
As is evident from the above description, among methods described in the above gazette, a surface processing for preventing the sunlight from being collected on the assistant reflecting mirror when it is applied to a parabolic antenna of the order of 10 m is only blasting and etching.
In the antenna panel 1 of a parabolic antenna for millimeter waves and sub-millimeter waves, in the case where the diameter is about 10 m, a mirror surface accuracy is required to be a very high accuracy of 10 xcexcm or less RMS. In many cases, in the antenna panel 1 of the parabolic antenna using a usual wavelength a little longer than the millimeter waves and the sub-millimeter waves, the mirror surface accuracy is required to be larger than 200 xcexcm RMS. In manufacturing the antenna panel 1 like this, a thin plate is deformed and fixed to ribs (reinforcing members), which are also deformed in advance, by means of welding, adhesion, riveting or the like. However, it is difficult to achieve a mirror surface accuracy higher than this level by this manufacturing method. Therefore, the mirror surface side needs to be formed by machining, which can produce a high surface accuracy.
Further, since the antenna panel is too heavy if it is a large block, the back surface thereof needs to be machined so as to thin the panel. If the panel is only machined to reduce its thickness, it has no sufficient rigidity and thus needs to have a structure in which reinforcing ribs are fixed to the back surface thereof. In such a manufacturing method, many panels about 1 m square are made and combined with each other to manufacture the antenna panel 1 of 10 m in diameter.
However, there is the following problem in the case where the surface processing of blasting or etching to satisfy the reflecting and scattering properties is applied to the panel described above.
First, the case where the panel is subjected to blasting will be described. Although the blasting method itself is not a problem, the surface subjected to blasting is deformed plastically by the blasting and is inevitably brought into an elongated state, thereby being deformed convexly. If the mirror surface (front surface) side and the back surface side are subjected to the same blasting, the deformation does not occur, but if the shape of the mirror surface side is very different from that of the back surface side, it is difficult to equalize the blasting conditions for both the surfaces. For example, if the back surface side is provided with the reinforcing ribs, the reinforcing ribs prevent the abrasive grains from hitting the back surface appropriately. If the wavelength is long, the amount of deformation is negligible but if the wavelength becomes as small as the sub-millimeter waves, the amount of deformation exceeds the required accuracy.
Next, the case where the panel is subjected to etching will be described. Since the etching removes the surface of the panel by several xcexcm, it can remove residual stresses in the layer where the quality is altered by machining. Therefore, if the residual stresses to be removed by the etching are different between the front surface side and the back surface side, stresses in both sides get out of balance, which causes the deformation of the panel.
FIG. 12 is a graph to show the deformation of a sample caused by etching in the case where the residual stresses in the front and back surface sides of the sample are different from each other, and shows the amount of deformation of the sample, which is made by shaving a plate of 10 mm thick and 250 mm square in the shape of # to a thickness of 2 mm, when the sample is etched by a caustic soda (NaOH, sodium hydrate). On the mirror surface side, the residual stress is small because machining conditions such as the depth of cut and a feed rate are not so severe, whereas on the back surface side, because considerably severe machining conditions are used to increase productivity, the residual stress is made large and thus the sample is greatly deformed by etching. Even in the case of using etching, this deformation is a big problem. Further, the surface roughness becomes large, depending on the etching conditions, which might scatter also radio waves to be observed.
In this respect, the deformation of the panel, which becomes a problem when the panel is subjected to blasting and etching, can be avoided to a certain extent by increasing the rigidity of the panel. For example, if the thickness of the plate forming the mirror surface is doubled, strength is increased by 8 times to greatly decrease the amount of deformation. However, increasing the rigidity of the panel increases also the weight thereof and thus there is a limit in this measure.
FIG. 13 is a characteristic graph to show the relationship between the surface roughness and the service frequency of the antenna panel. As shown in FIG. 13, in order to make the service frequency available up to as high a frequency as 2 THz, it is necessary to make the magnitude of the fine bumps and dips showing the surface roughness not larger than 5 xcexcm RMS. The service frequency is determined by the following equation
xe2x80x83Service frequency=speed of light/(surface roughnessxc3x9730)
Since the antenna system in the related art is constituted in the above manner, in the case where the panel constituting the antenna panel is subjected to blasting or etching so that the fine bumps and dips are formed on the panel, there is a problem that the amount of deformation of the front surface of the panel becomes large and exceeds a required mirror accuracy.
The present invention has been made to solve the problems described above. It is the object of the present invention to provide an antenna system provided with an antenna panel which can be used for radio waves in a high frequency range of from millimeter waves to sub-millimeter waves and has an excellent reflecting performance to suppress the collection of the sunlight and a high mirror surface accuracy, and a method for manufacturing the same.
An antenna system according to the present invention is provided with an antenna panel having fine bumps and dips formed on a mirror surface side, which is one of surfaces opposing to each other, by etching, wherein the fine bumps and dips scatter sunlight entering the mirror surface side of the antenna panel, yet regularly reflect radio waves having longer wavelength than the sunlight entering the mirror surface side of the antenna panel.
In the antenna system according to the present invention, the antenna panel has the fine bumps and dips formed on the mirror surface side by etching the mirror surface side by use of an acid solution.
In the antenna system according to the present invention, the surface roughness of the fine bumps and dips is in a range of from 0.5 xcexcm RMS to 5 xcexcm RMS.
In the antenna system according to the present invention, the antenna panel has fine bumps and dips having a chemical surface film formed thereon on the surface thereof.
In the antenna system according to the present invention, the antenna panel has fine bumps and dips having a chemical surface film formed thereon by alodine processing on the surface thereof.
A method for manufacturing an antenna system according to the present invention includes the steps of: finishing front and back surface sides of opposing surfaces of each of the constituent parts; masking the back surface side of each of the finished constituent parts and etching the front surface side thereof to form the fine bumps and dips; and combining the plurality of constituent parts subjected to the etching step in such a manner that their front surface sides form the mirror surface side of the antenna panel to assemble the antenna panel of a desired shape.
In the method for manufacturing an antenna system according to the present invention, the etching step is performed by etching the front surface side of each of the finished constituent parts by use of an acid solution.
In the method for manufacturing an antenna system according to the present invention, the etching step is performed by etching the front surface side of each of the finished constituent parts by use of an alkali solution and thereafter by an acid solution.
In the method for manufacturing an antenna system according to the present invention, the etching step is performed such that the surface roughness of the fine bumps and dips is made in a range of from 0.5 xcexcm RMS to 5 xcexcm RMS.
The method for manufacturing an antenna system according to the present invention further includes the step of applying a surface treatment for forming a chemical surface film to the front surface side of each of the constituent parts subjected to the etching step.
In the method for manufacturing an antenna system according to the present invention, the surface treatment step forms the chemical surface film by the alodine processing.
In the method for manufacturing an antenna system according to the present invention, the finishing step before the etching step forms the fine bumps and dips on the front surface side of each of the constituent parts by cutter marks produced when the front surface side of each of the constituent parts is subjected to machining.